There are predictions that in future rapid technological development could result in a significant shortage of paid work. A possible option currently debated by academics, policy makers, trade unions, employers and mass media, is a shorter working week for everyone. In this context, two important research questions that have not been asked so far are: what is the minimum amount of paid employment needed to deliver some or all of the well-being and mental health benefits that employment has been shown to bring? And what is the optimum number of working hours at which the mental health of workers is at its highest? To answer these questions, this study used the UK Household Longitudinal Study (2009-2018) data from individuals aged between 16 and 64. The analytical sample was 156,734 person-wave observations from 84,993 unique persons of whom 71,113 had two or more measurement times. Fixed effects regressions were applied to examine how changes in work hours were linked to changes in mental well-being within each individual over time. This study found that even a small number of working hours (between one and 8 h a week) generates significant mental health and well-being benefits for previously unemployed or economically inactive individuals. The findings suggest there is no single optimum number of working hours at which well-being and mental health are at their highest - for most groups of workers there was little variation in wellbeing between the lowest (1-8 h) through to the highest (44-48 h) category of working hours. These findings provide important and timely empirical evidence for future of work planning, shorter working week policies and have implications for theorising the future models of organising work in society.